The broad, long-term goal of this proposal is to understand the central role of glutamate and its related metabolites in retinal function. We will investigate the importance of glutamate, glutamine and gamma-aminobutyric acid and their relationship to carbohydrate metabolism and how these interrelationships are expressed in retinal neurons and Muller glial cells. Our working hypothesis is that glutamate provides an important source of carbons for energy production under aerobic conditions in the retina and that its metabolism is different in various retinal cells or layers. The major objective of this five year proposal is to identify the substrate sources and cellular sites of metabolic CO2 production in the rat retina Specific Aims proposed to accomplish the objectives are: 1. To test the hypothesis that CO2 production from exogenous glutamate requires glutamine synthesis and export by the Muller cells, its subsequent entry and conversion to glutamate in neuronal cells and further metabolism by: a) measuring the production of CO2 in whole retina from exogenous glutamine and glutamate in the presence or absence of glutamine synthetase inhibitors; and b) measuring glutamate metabolism and enzyme activities in monolayer cultures of rat and human Muller cells. 2. To determine the effects of activation of glutamate receptors by exogenous glutamate on energy production from glucose and on the metabolic pathways studied in Aim 1. Isolation of receptor- and substrate-dependent effects of glutamate will be achieved with the use of uptake blockers and receptor agonists and antagonists. 3. To determine the relative importance of glutamate transaminase, dehydrogenase, and decarboxylase because of the known heterogeneity in expression of these enzymes in the retina by: a) measuring their effects on CO2 formation from radiolabeled glutamate in the presence of specific enzyme inhibitors; and b) measuring the regional metabolism of glutamate and enzyme distribution in partial retinas, enriched in either outer or inner retinal layers. The measurements will include glycolysis, oxygen uptake, formation of carbon dioxide from isotopically labeled substrates, light-induced electrical activity, high energy phosphate levels, enzyme activities, leakage of lactic acid dehydrogenase, and morphological analyses. Our approach will provide new insights into the relationships between glutamate and retinal energy metabolism and the linkage between glutamate as an energy source and glutamate as an excitatory transmitter.